Commercial aircraft often include an on-board auxiliary power unit to provide electrical power and compressed air to various systems. Auxiliary power units are mostly used when the aircraft is on the ground, but they may also be used to provide pneumatic and electrical power during flight. When the aircraft is on the ground, the auxiliary power unit is a main source of power to drive the environmental control systems, the air-driven hydraulic pumps, and the starters for the engines.
Auxiliary power units require a certain amount of cooling air. In particular, auxiliary power units are lubricated with oil that is cooled by an oil cooler. In some systems, an active fan (i.e., with moving mechanical parts) pushes air across the oil cooler and through the compartment within which the auxiliary power unit is housed. The active fan is driven at high speeds by a complex shaft and gear assembly operatively connected to the auxiliary power unit. Because of the high operating speeds and numerous complex mechanical components, active fans may fail after extended periods of use. Fan failures have a negative impact on the reliability of auxiliary power units, which ultimately increases the cost of operating the aircraft. Accordingly, it has been the goal of auxiliary power unit designers to configure a passive cooling system that reduces or eliminates the number of complex moving mechanical parts.
One known passive system is described in U.S. Pat. No. 5,655,359. In the '359 device, a vacuum system passively cools both the oil cooler and the entire auxiliary power unit compartment. The system consists of two concentric nozzles positioned downstream of the auxiliary power unit turbine. An inner nozzle (or primary nozzle) flows high speed primary exhaust gas out of the gas turbine. An outer nozzle is positioned about the inner nozzle and is connected to a large circular plenum structure. The outer nozzle and plenum structure are referenced to the static air pressure of the auxiliary power unit compartment through one or more openings spaced around the plenum structure.
The combination of concentric nozzles, radial openings, and plenum structure functions as an aspirator, or pumping device, commonly called an eductor system when applied to auxiliary power unit installations. The outer nozzle provides a passage for expulsion of compartment air out the tail cone of the aircraft. The relative velocity difference between the gas fluid in the inner and outer nozzles creates a depressed pressure region in the eductor. This provides a "pumping" action to draw compartment air into the eductor and into the outer nozzle to combine with the primary turbine exhaust gas. By placing the oil cooler adjacent the eductor, compartment air will flow through the oil cooler passages, thus cooling its internal oil.
The '359 arrangement further includes an air intake duct extending between a forward-facing ambient air opening and the auxiliary power unit. The opening is typically closed when the auxiliary power unit is off and open when on. In the open position, a door protrudes above the aircraft skin. During flight, the auxiliary power unit is ram-fed combustion air by the ambient airflow coming into the opening and through the intake duct. An inlet scoop is positioned inside the intake duct to separate and route a portion of the ram-fed airflow into the auxiliary power unit compartment. For ground and flight operation, the pumping action of the eductor system draws the scoop air into the compartment, through the oil cooler and out the outer exhaust nozzle. This dedicated airflow is utilized for cooling various auxiliary power unit components, such as the oil cooler, as well as maintaining an acceptable auxiliary power unit compartment air temperature.
While the '359 system is effective in reducing the overall number of moving parts, it has the disadvantage of requiring a relatively large opening in order to provide sufficient air to the auxiliary power unit for combustion and air to the auxiliary power unit compartment for cooling. The large opening requires a large, operable door, which can undesirably cause aerodynamic drag and other performance penalties during flight. In addition, the overall large size of the intake duct makes it undesirably heavy.
Thus, a need exists for a passive auxiliary power unit oil cooling system that is lighter weight and more aerodynamically efficient. The present invention is directed to fulfilling this need.